dart2js: Type inferrence: Account for implicit null test in '??' and '??='.

pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart

Index: pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
diff --git a/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart b/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
index 4a03bafed62e2f1b15f56ab4ae039827043fb166..dded10cf912cfde83b588831ba2dcb5e783042ea 100644
--- a/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
+++ b/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
@@ -337,6 +337,16 @@ class TypeInformationSystem extends TypeSystem<TypeInformation> {
     }
   }
 
+  TypeInformation narrowNotNull(TypeInformation type) {
+    if (type.type.isExact && !type.type.isNullable) {
+      return type;
+    }
+    TypeInformation newType =
+        new NarrowTypeInformation(type, dynamicType.type.nonNullable());
+    allocatedTypes.add(newType);
+    return newType;
+  }
+
   ElementTypeInformation getInferredTypeOf(Element element) {
     element = element.implementation;
     return typeInformations.putIfAbsent(element, () {